1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to toner concentration monitors used in copiers, printers, and like devices.
2. Description of the Prior Art
Various forms of electrostatographic apparatus, such as electrophotographic copiers and printers and electrographic stylus and pin recording devices, use dry powdered toner to develop the latent images created by the particular exposure or writing process. Frequently, the developer is a mixture of carrier particles and toner particles. In order for the apparatus to produce quality hard copy outputs, it is necessary to maintain the concentration of the mixture within relatively narrow limits. Thus, the ratio of toner to carrier particles in the mixture must be accurately maintained.
Before toner concentration can be maintained within tight tolerances, there must be a reliable and efficient method of measuring the toner concentration. Various types of systems have been used for this purpose according to conventional practices. One type of system measures the density of a test patch developed on the latent image-bearing member of the apparatus. Another type of prior art system measures the magnetic properties of the developer and equates that to a ratio of non-magnetic toner particles and magnetic carrier particles. Still another type of system optically measures the amount of light reflected from the development mixture to determine the carrier-toner ratio. This type of system relies upon the fact that the carrier particles and the toner particles have different light reflectance values or coefficients. Thus, when the concentration ratio changes from the desired value, the reflectance of the mixture either increases or decreases. Usually, the toner is darker than the carrier and decreasing the ratio of toner to carrier causes an increase in the light reflectance from the developer.
U.S. Pat. No. 4,833,506, issued on May 23, 1989, discloses a system for controlling the toner density in a copying machine. In this patent, a test patch is developed on the photosensitive drum and the density of the patch is measured by an optical sensor. U.S. Pat. No. 4,326,646, issued on Apr. 27, 1982, also discloses a toner concentration control system which uses a developed test patch. In this patent, the gain of the detection circuit (FIG. 4a) is changed by a system controller which is connected to the feedback branches of the circuit amplifier. The detected signal is applied to one input of an operational amplifier and the other input is used for controlling the effective gain of the amplifier. In both these patents, the monitoring systems are not measuring the carrier-toner concentration directly, as substantially no carrier is on the test patch.
U.S. Pat. No. 4,572,102, issued on Feb. 25, 1986, is indicative of systems which measure the developer directly in the developer container. With such systems, the developer mixture being measured usually contains both carrier and toner particles as contrasted to test patch systems which predominantly measure only toner particles. In this particular patent, the developer contains two different types of toner which are separately measured for light reflectance. The measured values are compared to each other (FIG. 4) and ultimately control the replenishment of toner in the developer. A level sensor 50 is used to measure the amount of developer in the container.
U.S. Pat. No. 4,155,638, issued on May 22, 1979 to the same assignee as the present invention, discloses a toner concentration monitor which measures the light reflectance of the developer contained in the developer container, as does the present invention. Both direct and reflected light sensors are used to measure the reflectance of the mixture in the container. The two signal processing circuits 69 and 72 in the referenced patent are connected to a digital computer which determines if replenishment is necessary.
Some development mixtures contain carrier particles which are more reflective than others. Highly reflective stainless steel carriers are used in developers which include both metallic particles and toner particles which are intended to be deposited on the developed image. This type of developer is used when the produced image is to contain magnetic particles which can aid in scanning text and numbers contained in the image produced on the hard copy. The difficulty with such developers when used with the conventional toner concentration monitors is the fact that the increased reflectance of the carrier causes the detector circuits to operate outside normally designed ranges. This is attributed to the fact that the reflectance of the developer is much higher than normal even when the toner concentration is optimum. The increased reflectance can be compensated for by changing the amplifying ratio or gain of the circuits, or by increasing the range of the A/D converter through which the measured reflectance is applied to the computer circuitry. However, this also changes the response and accuracy of the system. If, for instance, the gain needs to be lowered significantly to compensate for the highly reflective carrier, the system will have difficulty responding fast enough to correct large errors in toner concentration. If the gain needs to be raised to compensate for the reflectance of the carrier, the system may become unstable and respond by overreacting to small changes in toner concentration.
To increase the versatility of a toner concentration control system, it is also necessary to be able to adjust the system for different toner concentration aim or desired levels without changing the stability and/or response of the system. Therefore, it is desirable, and an object of this invention, to provide a universal toner concentration control system which can be used effectively with a wide range of toner and carrier materials, including highly reflective carrier particles, and which enables adjustment of the aim toner concentration without changing the sensitivity of the toner concentration monitor.